1. Field of the Invention
This invention relates to heaters, and, more particularly, to a novel convection section assembly for heaters, and especially fluid process heaters.
2. Description of the Prior Art
Convection zones are normally used in direct-fired process heaters to obtain low temperature differentials between the process fluid inlet and flue gas exit. Extended surface tubes, i.e. tubes with fins or studs formed about or in their outer surface, are normally used in such convection zones, since the heat transfer coefficient on the outside (flue gas side) of the tubes is normally much lower than on the process side (inside) of the tubes.
The tubes are normally disposed longitudinally in the convection sections and are supported near the ends and at intermediate points by tube sheets (plates or castings having openings large enough to pass the tubes and attendant extended surface). Such an arrangement, however, limits the minimum spacing of the tubes to that which will leave sufficient material between the openings to permit sound construction and does not permit close placement of tubes to obtain high velocity and good heat transfer coefficient on the flue gas side. A variety of different devices have been proposed for tube supports, but these are all internal devices and therefore limit the flue gas temperature to levels that can be met by available metallurgy. The above deficiencies are illustrated by the following U.S. patents.
In U.S. Pat. No. 3,274,978, for example, to Palchik et al, having a common assignee with the assignee of the present invention, there is disclosed, in one embodiment, a vertical tube-type process heater having a single convection zone disposed above a plurality of radiant heating zones and provided with one or more sections of horizontally-disposed conduits. Furthermore, in U.S. Pat. No. 1,919,192 there is described a serpentine heating coil for which intermediate and end supports are needed. Consequently, in accordance with this arrangement, all such supports have to be internal, a factor which thereby limits the width of the convection section. In U.S. Pat. No. 3,929,189, moreover, the use of a vibration dampening device is described which clamps tightly against the tubes. In this patent, the supports for both the tubes and the dampening device as well are internal, thereby effectively limiting the width and flue gas temperature of the convection section.
In another patent, U.S. Pat. No. 4,246,872, another variety of tube support is described. In this patent, a structure is disclosed which accommodates differential expansion over the tubes by allowing the internal tube supports to swing or move, such supports being located in the flue gas stream.
However, in the design of both the prior art convection section assemblies for process heaters and of the tube supports utilized therein, there are no means provided to enable such convection assemblies to be of unlimited width and no means provided to accommodate unlimited flue gas temperature.
In addition, there are other considerations. For example, in accordance with the principles of normal construction, the width of the convection section is usually restricted to the limits of sound castings and construction. When the volume of fluid flow on the process side is large, either a large number of streams or large tubes must be used. Double or multiple serpentine coils may be used; however, this results in some tubes and return bends being inaccessible for repair or maintenance, difficulty in connecting to inlet and outlet manifolds, and some cross-flow effect which reduces over-all heat transfer effectiveness. It should also be noted that, when the tube diameter is doubled, only one-half the number of tubes may be installed in the same width. Moreover, the flow area of each tube is four times as large; therefore the flow area is doubled. Thus, the use of larger diameter tubes is not a satisfactory solution since such tubes have lower heat transfer coefficients for the same flow conditions, thereby requiring more surface area. Also, since the volume-to-surface ratio increases directly with tube diameter, and since some process services are sensitive to residence time, these factors would also be adversely affected by the use of large diameter tubes.
Thus, the known convection section assemblies are all characterized by the above significant deficiencies. The present invention now provides the means for overcoming such deficiencies; it provides a convection section assembly having only external fixed support means which in and of themselves can be of unlimited length, thereby: (a) permitting the convection assembly itself to be of unlimited width; (b) enabling the tube sheets supporting the tubes of the convection assembly to expand in a lateral direction; and (c) enabling the tube supports to be fixed and the tubes themselves to slide. Thus, with only such external fixed support means, which can be refractory lined, there is no practical limit on the flue gas temperature.